In the prior art, hybrid substrates, typically silicon-on-sapphire (SOS) substrates based on support substrates of sapphire having characteristics including good insulation, a high thermal conductivity, and a low loss in the radio-frequency band are used to construct devices for radio-frequency applications.
For the manufacture of SOS, heteroepitaxial growth of silicon on sapphire substrates is well known. This method has the drawback that many defects generate owing to the difference in lattice constant between sapphire and silicon (see Non-Patent Document 1, for example).
In view of the above problem, there was proposed a method comprising the steps of bonding a hydrogen ion-implanted silicon substrate to a sapphire substrate, making the hydrogen ion-implanted layer brittle, and separating the silicon substrate for thereby transferring a single crystal silicon thin film onto the sapphire substrate (see JP-A 2010-278337: Patent Document 1, for example).
For the SOS substrate, since the sapphire substrate and the silicon layer have approximate lattice constants, often substrates having a face orientation of R face are used as the sapphire substrate and substrates having a face orientation of (100) face are used as the silicon substrate. Since the sapphire substrate of R face is anisotropic, anisotropy appears in the coefficient of thermal expansion and the coefficient of thermal conduction, and an in-plane variation occurs in the threshold voltage after MOS-FET fabrication. These problems are pointed out in WO 2011/077608 (Patent Document 2).
Therefore, Patent Document 2 describes that a sapphire substrate of C face is low anisotropic and effective for improving the in-plane uniformity of device properties. Also pointed out therein is the advantage that since C-face sapphire substrates are often used in optical devices such as LEDs, they are available at lower cost than R-face sapphire substrates and contribute to the cost reduction of SOS substrates and devices using the same.